It is known that levelers are designed to correct or considerably reduce any defects in metal sheet that result from various manufacturing stages (rolling, coiling, heat treatments). Mention may be made, for example, of developable defects (initial bend, “tile”) or nondevelopable defects (defects “along the edge” or “along the center”).
It will be recalled that the principle of cold leveling consists schematically in converting a geometrical defect into a system of variable residual strains within the thickness by means of alternating bending stresses. The sheet or strip to be leveled thus passes through a stand formed from an assembly comprising at least two series of lower and upper rolls placed facing each other. The two series are arranged so as to be approximately parallel to each other and perpendicular to the run direction of the strip. When the sheet passes between these rolls, it undergoes partial plastic deformation in bending in one direction and then in the opposite direction. The amplitude of the bending progressively decreases because the imbrication of the rolls decreases upon going toward the exit of the leveler. Consequently, it is possible to manufacture products of excellent flatness and with a very low level of residual stresses, capable of meeting certain applications in the fields of metal furniture, domestic electrical appliances and the automobile industry.
The ever tighter product tolerances, in terms of flatness or level of residual stresses, imposed by users, require ever better control of the mechanical behavior of levelers, without which their operation would remain uncertain. At this stage of the description, it seems worthwhile, in order for the various adjustment parameters to be better understood, to describe the main components of a multi-roll leveler.
For this purpose, FIG. 1 shows, in schematic longitudinal section, a conventional leveler comprising a series of lower rolls 11 (identified by 11a to 11n, there being ten of them in this particular case) and a series of upper rolls 12, identified by 12a to 12m, supported by a lower beam 13 and an upper beam 14, respectively. The metal strip 10 is driven through the leveler along the direction indicated by the arrow F. The rolls 11 and 12 are mutually imbricated to an extent that decreases along the run direction of the strip. Thus, the strip is significantly deformed by being bent between the entry rolls 11a, 12a, 11b, but very little in the exit rolls 11m, 12m, 11n of the exit zone of the leveler. Consequently, the initial geometrical defects in the strip are converted by plastic deformation into a system of residual strains whose amplitude decreases with that of the bending imposed by the rolls.
FIG. 2 shows schematically known means for adjusting the imbrication of the rolls: the “tilt” characterizes the inclination of the upper beam 14 relative to the lower beam 13 in the run direction of the strip. The lower beam is considered as a reference plane. The beam 14 is supported on an upper frame 15 by adjustment assemblies 16a, 16b, 16c and 16d, for example of the screw-nut type with an angle gear or other technologies.
The adjustment assemblies according to the aforementioned example, of the screw-nut type, are actuated by the motors 19a and 19b by means of drive shafts 17a and 17b. The couplings 18a and 18b are used to temporarily decouple the adjustment assemblies that they connect, so as to be able to adjust the transverse parallelism (or “dislocation”) between the upper and lower rolls, both on the entry side and on the exit side of the leveler. The imbrication of the rolls is then adjusted by means of the motors, which simultaneously drive the adjustment assemblies at the entry or exit of the leveler.
The dislocation has to be removed by a considerable number of operations on the machine. The tilt is adjusted in a standard fashion in order to modify the imbrication of the rolls, in particular according to the characteristics of the leveled strip.
FIG. 3 shows schematically a conventional leveler in a front view, which illustrates the means available for compensating for the bending of the rolls under load. This is because the reaction forces during strip leveling cause the rolls to deform. To compensate for such deformation, the leveling rolls are supported by stages of support and counter-pressure rolls, ramps or rollers.
This assembly is mounted in a frame called a “cassette” placed on a set of “counter-pressure ramps” that are independent and height-adjustable, these ramps being distributed in the transverse direction of the leveler. FIG. 3 illustrates an example of eleven rows of counter-pressure means 21 for compensating for the bending of the rolls 11. The lateral movement of the rolls is limited by the bearings 20. The vertical position of these ramps can be adjusted, for example by means of adjustable tapered wedges 22.
FIG. 4 shows, in an intentionally exaggerated manner, the deformations created by these counter-pressure ramps on the lower rolls by means of a more or less important vertical displacement of the ramps. The deformations may be produced on the rolls under no load or under load.
FIG. 5 shows a known example in which the height of these counter-pressure means can be adjusted by means of tapered wedges 23 interposed between the support rolls 11 and a rigid lower frame 15′. The relative displacement of the tapered wedges is provided by a cylinder 24 and can be measured, for example, by a position sensor 25.
In the case of a leveler comprising 5 to 6 levels of superposed rolls (case not shown here), eccentric rollers are also present, these bearing on intermediate rolls and making it possible to adjust the clamping of the entry roll 12a and the exit roll 12m. 
Thus, the overall adjustment of a leveler involves many parameters, and in particular:                the adjustment of the dislocation (transverse parallelism between the upper and lower rolls) carried out for example by screw-nut adjustment assemblies, or counter-pressure ramps;        the adjustment of the roller imbrication at the entry and at the exit of the leveler by tilting the beam;        the adjustment of the counter-pressure means in order to compensate for the bending of the rolls under load; and        the tension in the strip.        
To be able to adjust the leveler according to the characteristics of the strip, it is therefore necessary to calibrate or initialize said leveler. This amounts to determining the suitable base adjustments of the leveler in order to obtain the intended effect. It is also desirable to know the adjustment values controllable by the available means (especially imbrication adjustment, counter-pressure rollers height adjustment) and also the amount of play, spring and bending of the rolls during leveling. It is thus possible to take account of these parameters in adjusting the leveler.
To obtain good product flatness therefore requires:                for the entry and exit gaps of the leveler to be precisely calibrated, for example to ±0.05 mm;        to check the absence of parasitic tilt or dislocation in beams theoretically parallel from the standpoint of the operator; and        to precisely calibrate the height of the counter-pressure means, for example to ±0.02 mm.        
Now, at the present time, the leveling operations, and in particular the calibration, involve a certain amount of empiricism, for several reasons:                the preadjustment settings or charts indicated by the manufacturers may prove unsuitable;        regular checking of the leveler calibration is often performed by the operators, using a ground bar or a round product of calibrated diameter introduced into the gap in order to check the value when the leveling rolls come into contact with this bar. This operation, carried out in the absence of load, therefore does not guarantee that there will be a precise gap when the leveler is under load (clamping onto a product) since the play and spring of the machine are not taken into account in this method. The precision of this calibration method is difficult to quantify, as it is obviously dependent on the sensitivity and the experience of the operators, and its reproducibility is not guaranteed.        
However, a method has been proposed (“Modeling of the leveling process and applications to heavy plate mills and strip finishing mills”, METEC Düsseldorf 1994) for characterizing levelers under dynamic load and allowing them to be calibrated. This method relies on the use of reference sheets of different formats, which include strain gages placed in line with each counter-pressure means.
Although this method using instrumented sheets is perfectly suitable for defining the initial adjustments of a leveler, it is however poorly suited to regularly monitoring its proper operation. This is because its use requires the machine to be stopped for several hours and a skilled operator has to work on the machine, with sophisticated measurement means, thereby greatly penalizing productivity. In addition, in the case of large levelers, the size of such sheets and the difficulty of manipulating them become a not insignificant problem.